The Misconception Series #2 - Aluminum-Framed Storefronts

Contributed by Elias Saltz

​As a consulting specifier, my clients come for my expertise.  To bolster my knowledge, I frequently find myself in conversations with product reps, talking about the nitty-gritty technical aspects of their products.  These conversations delve into a far deeper level of detail than I would previously get when I was a ‘normal’ design architect and project manager. Over the course of those conversations, I am occasionally surprised that things I thought I knew a lot about were based on misconceptions.  In fact, even things that I considered “common knowledge” have been shown to be wrong, or at least over-simplifications.  Armed with accurate information, I can pass correct technical advice on to my clients, hopefully dispelling those misconceptions one person at a time, one project at a time. 
 
Misconceptions can be found across the spectrum, in every product category and in every MasterFormat number.  I thought it would be fun and enlightening to ask my go-to reps in a wide variety of product categories to tell me the biggest and most common misconceptions they hear as they work with designers and architects, and present their responses here.  In each post, I’ll relate my discussion with reps in one category or one MasterFormat header. 
 
The reps I chose to approach for this post, Kurt Wenzel from YKK AP and John Stelter from EFCO Corporation, are both active and involved CSI members that I’ve come to know well over my career.  I consider them my trusted advisors when it comes to questions about their companies’ lines of fenestration products.  I’m not promoting their products over their competitors’ - it’s far more about the individual reps than the companies that they work for.
 
08 43 13 - Aluminum-Framed Storefronts
 
Introduction to Storefronts:
 
Webster defines storefront as “The front side of store or store building facing a street.” The use of storefront products dates all the way back to the 1930’s, and the systems of today have changed very little from the original design. The design intent of the storefront sash and glass originally was to allow for shopkeepers to display their wares to pedestrians who would pass by their stores.  They aimed to entice them to stop and look with the hopes of attracting them inside.
 
Aluminum-framed storefronts are basically extrusions of aluminum that are fabricated and assembled to allow for glass (or other infills) to be installed into the system, providing a see-through weather barrier between the inside and outside of the building.  The extrusions are normally 1-3/4 to 2 inches wide by 4 to 4½ inches deep; systems 6 inches deep are available from some manufacturers.  Systems intended for use on buildings’ exteriors usually are fabricated with a thermal break lined up with the center of glass.  The thermal break reduces heat energy loss through the system, preserving energy and minimizing condensation.  That thermal break is omitted when the storefront is located on the interior, such as in a vestibule. 
 
With storefront systems, the entire extrusion is structural, there are no non-structural pressure caps or decorative covers like there are in curtain wall systems. Multiple configurations are available, and all are conceptually equivalent, other than the plane of the glass.  Configurations include structural glazed, front, center, and rear glazed systems.
 
CSI’s Specifier Practice Group recently held a webinar session discussing how storefronts, windows, curtain walls and window walls are made and how they’re distinguished from one another in performance and in their use.  The video of that webinar is available here.

Now, some misconceptions. I asked Kurt and John this question:

“When you think about the questions and comments you hear from design professionals across all levels of experience, what misconceptions about aluminum-framed storefront systems do you find that you most commonly have to dispel?”
 
“I can use storefront for this 20-foot tall wall.  It’ll work great!”

Today’s manufacturers and suppliers face many challenges from design professionals with respect to how storefronts are being used. They’re being pushed from typical performance issues of air, water and structural to ever increasing demands for better energy performance. Along with these performance requirements, larger opening sizes, both in width and height of a storefront system, are being sought.

As openings grow larger in width and height, architects seek to continue to use storefront systems for their ability to keep costs in line and deliver the aesthetes they seek. However, these larger openings can pose problems if not carefully reviewed by the manufacturer and supplier. Storefront systems are typically used to heights of 120 inches or lower and widths of 36 inches or less.  Now they’re being called upon to reach heights of over 144 inches, 60-inch-wide single panes of glass, and used in multi-floor applications. These new designs not only put stress on the storefront mullions to withstand design pressures, but also, due to these larger openings, water infiltration becomes a concern if not addressed properly. Air, water and structural requirements must be considered as one. They all work hand in hand. If one fails, the system fails and Owners are left with costly and time-consuming repairs.

Like many other things, there is no good rule of thumb for the width and height restrictions of storefront. Some key questions to be answered to determine if storefront should even be used are: What’s the vertical to vertical mullion spacing of your elevation? How tall is the opening? What’s the design pressure or wind speed? What’s the overall application, single story or multi-story? What type of project is it? Answering these questions can lead to the proper selection of the right system.  The best person to address these questions is the manufacturer’s rep.  Most storefront manufacturers also manufacture windows and curtain wall systems and will steer architects to the most appropriate system.
 
“I can use storefront instead of windows for punched openings high up in my building.” 

Storefront is typically used in one and two-story buildings because its wind load resistance is limited to about 40 pounds per square feet (psf).  Windows, by comparison, can be rated over 100 psf. The taller the building, the greater the design pressures that occur to that building, so taller buildings require a better performing system.  For the 3rd floor and above, consider a window, window wall or curtain wall system rather than storefront.  Additionally, water-resistance performance is limited at higher design pressures.
 
"But how can water performance be limited with a storefront when it’s all fixed glass?"

While storefronts are fixed glazing units, they’re designed to control water, not to be waterproof.  The issue with water performance has to do with how the storefront system is weeped or drained.  Storefronts work as a "gutter & downspout" system.  The positive and negative pressures that occur on a building during a storm will cause the storefront glass panes to oscillate or push away from the glazing gaskets allowing water to penetrate the system.  The horizontal mullions act as the gutter, collecting water and allowing it to run down to the vertical members, which act as the downspout.  The verticals run to the sill flashing, which catches any water and lets it escape through a series of weep holes.  It’s extremely important to install the sill flashing correctly, or the water may become trapped, or even sucked into the building.  The design professional must ensure that the flashing is properly detailed and specified: anchor screws/bolts need to be covered in a tooled sealant.  End dams need to be used at the ends of the subsill extrusion. Horizontal mullions need water deflectors to help guide the water down the vertical. 

Storefronts are not pressure equalized systems, like are curtain walls, so they rely on gravity and the size of the sill "tank" and properly-placed and clear weeps for drainage.
 
"My storefront leaks so I am calling the manufacturer!" 

Manufacturer’s reps will go out to investigate if the fabricator/installer can’t figure out why the system is leaking. That being said, manufacturers actually have very little to do with a leaking storefront system.  Generally, storefronts are referred to as “stick built” systems.  The manufacturer only extrudes the framing members.  A fabricator buys the extrusions in stock lengths, cuts, fabricates, drills, assembles, seals and glazes the system, normally on site.  The manufacturer will provide a set of installation instructions to show the glazing subcontractor how to fabricate and seal the system.  They require that fabricators follow the instructions or the warranties won’t be honored. If your storefront system has a leak, your first call should be to the glazing sub and get them to identify the cause. 
 
“Storefront systems are anchored at the head and sill only.”

Like most topics, this is partially true, but in reality, is a bit more complicated. In a run of storefront, the only attachments are located at the head and sill, but the terminating jambs are fastened to the wall substrates in order to transfer wind loads to the structure.  In single pane storefronts, anchorage to jambs may also be required to accommodate the expected load. 
 
Conclusion

As with any complicated topic, there is always more detail than can be posted in a ‘most common misconceptions’ discussion, and more to be learned.   When it comes to working with construction products and systems, all have complicated qualifications regarding their use.  Experienced product reps have seen and helped solve many real-life problems that have arisen through incorrect product selection, bad detailing and poor specifications.  They are a critical resource that can help us update our knowledge and dispel our misconceptions.
 
My thanks to John and Kurt for participating in this project.